1. Field of the Invention
The present invention relates to a photoelectric conversion device having phototransistors and particularly, to a photoelectric conversion device with improved photoelectric conversion characteristics and residual image effect, and reduced fixed pattern noises.
2. Related Background Art
One example of conventional photoelectric conversion devices is disclosed, for example, in Japanese Patent Laid-open Gazettes from 12579/1985 to No. 12765/1985. FIG. 13A is a plan view showing the pattern of a photoelectric conversion device described in the Publications, FIG. 13B is a sectional view along line I--I of FIG. 13A, and FIG. 13C is an equivalent circuit of a photosensor cell of the photoelectric conversion device.
Referring to FIGS. 13A to 13C, each photosensor cell of the photoelectric conversion device is constructed of an n-type silicon substrate 101 and electrically isolated from adjacent photosensor cells by an element isolation region 102 made of, for example, SiO.sub.2, Si.sub.3 N.sub.4, polysilicon or the like.
Each photosensor cell has the following constitutent elements: A p region 104 is formed by doping p-type impurity on an n.sup.- region 103 of low impurity concentration formed by the epitaxy method or the like. An n.sup.+ region 105 is formed in the p region 104 by the impurity diffusion method, the ion implantation method or the like. The p region 104 and n.sup.+ region 105 serve as the base and emitter of a bipolar transistor, respectively.
Formed on an oxide film 106 deposited on the n.sup.- region 103 with the above regions is a capacitor electrode 107 of predetermined area which faces the p region 104 with the oxide film 106 interposed therebetween and forms a capacitor Cox. The potential of the p region 104 in a floating state is controlled by a pulse voltage applied to the capacitor electrode 107.
The photosensor cell is constructed further of an emitter electrode 108 connected to the n.sup.+ region 105, an interconnection 109 for reading a signal via the emitter electrode 108 and sending it to an external circuit, an interconnection 110 connected to the capacitor electrode 107, an n.sup.+ region 111 of high impurity concentration formed on the substrate 101, and an electrode 112 supplying a potential to the collector of the bipolar transistor.
Next, the fundamental operations of the photosensor constructed as above will be described. First, assuming that the p-base region 104 of the transistor is at a negative potential and at a floating state. Upon incidence of light 113 to the p region 104, carriers corresponding in amount to the incident light are accumulated in the p region 104 (accumulation operation). The base potential changes with the accumulated charge so that the emitter-collector current is controlled. Thus, an electric signal corresponding to the incident light quantity is read out of the emitter electrode 108 at a floating state (readout operation). In order to remove carriers accumulated in the p region 104, the emitter electrode 108 is grounded and the capacitor electrode 107 is applied with a positive refresh pulse voltage. Upon application of a positive refresh pulse voltage, the p region 104 is forward biased relative to the n.sup.+ region 105 to thereby remove the accumulated carriers. After the refresh pulse falls, the p region 104 resumes a negative potential and hence a floating state (refresh operation). The above accumulation, readout and refresh operations are repeated.
Briefly stating the above proposed method, light-induced carriers are accumulated in the base p region 104 to control a current passing through the emitter and collector electrodes 108 and 112 in accordance with the accumulated charge quantity. The accumulated carriers are read after amplifying them by the amplification function of each cell, thereby achieving a high output and sensitivity, and less noise.
The potential Vp of the base with light-induced carriers accumulated therein is given by Q/C, wherein Q represents the charge of carriers accumulated in the base region, and C represents a capacitor coupled to the base region. As apparent from the above equation, the values of Q and C both become small as the cell size becomes small due to high integration. Thus, the light-induced potential Vp is maintained substantially constant. Therefore, the above proposed method may become useful in the future for a means of obtaining a high resolving power.
A change in base potential V.sub.B while a positive refresh pulse voltage is applied to the capacitor electrode 107 can be given by the equation (Cox+Cbe+Cbc)(dV.sub.B /dt)=-I.sub.B, where Cbe is a capacitance between base and emitter, Cbc is a capacitance between base and collector, and I.sub.B is a based current.
FIG. 14 shows a change with time in base potential V.sub.B while a positive refresh pulse voltage is applied.
Referring to the graph, the initial base potential when a positive refresh pulse voltage is applied changes with the magnitude of an accumulated voltage Vp. Namely, the base potential which takes a negative potential at an initial condition changes in the positive direction by an accumulated voltage Vp during the accumulation operation, and upon application of a positive refresh pulse voltage to the capacitor electrode 107, the initial base potential rises by the amount corresponding to the accumulated voltage Vp.
As seen from the graph, the period while an initial base potential is retained differs depending upon the magnitude of the initial base potential. After that period, the base potential V.sub.B, however, gradually drops irrespective of its initial potential. Therefore, the base potential may be maintained at near zero volt irrespective of the accumulated voltage Vp on condition that a sufficiently long refresh time t is used. As a result, it is possible to make the base potential V.sub.B return to a predetermined negative potential when the positive refresh pulse voltage falls.
In practice, however, the refresh time is limited so as to achieve a high speed operation. For example, a refresh operation terminates when the base potential V.sub.B becomes V.sub.K assuming the refresh time t=t.sub.0. In such a case, even if the base potential VB includes a residual potential at the end of a refresh operation, it is possible to make the base potential V.sub.B return to a predetermined negative potential and hence to an initial negative potential at the trailing edge of a positive refresh pulse voltage, so long as the base potential V.sub.B is at a constant potential of V.sub.K at t=t.sub.0.
However, repetitive refresh operations of a conventional photoelectric device results in a gradual drop of a residual potential V.sub.K, and hence results in a non-linearity of the photoelectric conversion characteristics and a residual image effect. These phenomena will be clarified in the following:
Referring back to FIG. 14, it is now assumed that the initial base potential of a cell under high illumination is 0.8V, and under low illumination 0.4V. In this case, after a refresh operation for a time period t.sub.0, the base potential V.sub.B of the high illumination cell takes a predetermined residual potential V.sub.K, whereas the low illumination cell takes a residual potential V.sub.1 slightly lower than V.sub.K. When the refresh pulse rises in such a state, the base potential V.sub.B of the low illumination cell takes a negative potential lower than the initial potential from which the following accumulation and readout operations start. Consequently, as the refresh operation repeats under such low illumination, the base residual potential gradually lowers. If the lower illumination cell with a lowered residual potential is changed to operate in high illumination, an output lower than that corresponding to an incident light quantity will be obtained. Namely, non-linear photoelectric conversion characteristics and residual image effect occur.
The reason for this may be attributable to the deficit in carriers (holes) due to the recombination of holes in the base region. As the low illumination condition continues during which lost carriers (holes) cannot be replenished, the non-linear photoelectric conversion characteristics and residual effect becomes conspicuous.
To solve the above problem, a method of removing carriers in the base regions has been proposed and filed by the present applicant. According to this method, a MOS transistor 113 shown by dot-lines in FIG. 13C is turned on when a refresh operation is initiated to couple an initial base potential as shown in FIG. 14 to the base region and remove the carriers.
Particularly, according to an aspect of the above-proposed photoelectric device, in a phototransistor having a semiconductor region including two main electrode regions and a control electrode region formed between the two main electrode regions, and a capacitor for controlling the potential of the control electrode region in a floating state and accumulating carriers produced by an electromagnetic wave incident to the semiconductor region while controlling via the capacitor the potential of the control electrode region in a floating state, the photoelectric conversion device comprises: first control means for removing said carriers by controlling the potential of said control electrode region via said capacitor; second control means for removing the carriers of said first control means; and means for maintaining, immediately before the carrier removal operation by said second control means, the potential of said control electrode region at a constant potential for a predetermined period using a switch connected to said control electrode region.
By providing, at the control electrode region, means for maintaining the potential of the control electrode region at a constant potential immediately before the start of a carrier removal operation, it becomes possible to set the potential of the control electrode region at a desired value when the carrier removal operation ends. Thus, a non-linearity of photoelectric conversion characteristics and residual image effect can be improved.
Although the above photoelectric device is very effective in improving the non-linearity of photoelectric conversion characteristics and residual image effect, there still remain some problems caused by the capacitor itself formed above the control electrode region for controlling the potential thereof.
Particularly, first the capacitance C coupled to the control electrode region increases by Cox so that the potential Vp to be generated at the control electrode region lowers. Second, dimension tolerance of capacitors Cox in a plurality of phototransistor arrays results in scattering of the light-induced potential Vp.
More in particular, the base potential after the refresh operation is determined depending upon a constant K which is: EQU K=Cdg/(Cox+Cbe+Cbc)
where Cbe represents a base-emitter capacitance of a bipolar transistor, Cbc represents a base-collector capacitance, and Cdg represents a gate-drain capacitance of the MOS transistor 113. As a result, if these capacitance values scatter for each element, the base potential after the refresh operation will also scatter, which results in fixed pattern noises of a photosensor cell array.
Third, the capacitor Cox is generally constructed as of MOS structure. Control of the interface between SiO.sub.2 and Si is difficult in most cases and in addition, the condition at the interface changes with an electric field, which is another reason of scattering of elements.